I'm thinking about purchasing a 28' enclosed trailer. I'll be using it for a race car and I'm wondering about sizing. It seems like 28' is the biggest you can practically go without resorting to a gooseneck or 5th wheel trailer. Does anyone with enclosed trailer experience have thoughts on that? I use a variety of tow vehicles and I cannot count on anything other than a conventional tag along hitch.

Other than 110/12v internal wiring, generator storage, the usual brake considerations, and some cabinets up front, what should I be looking for? I just don't want to sink a few grand into a trailer and realize life would be way better with feature X the next week.

Load rating. Make sure that the trailer has the load rating you need to carry all the extra stuff. Many enclosed trailers have a fairly low weight capacity. The enclosure subtracts from the overall capacity of the trailer. Be sure you have a good idea of what all you want to carry. You will always stow more stuff in the trailer than you originally plan. Don't forget if you want to carry fresh water, fuel, camping equipment, spare parts, 2 vehicles?, etc... You will always underestimate what your stuff weighs.

Are you using it for a trail rig or only a race car? A trail rig on 35's will barely fit into a standard height trailer.
Get it insulated and finished inside.
Get the 110V flourescant light package.
Decide where you want your work bench/ utilities and specify where you want your 110v plugs.
Get the RV door latch on your side door.
If you plan to tow it with an RV, get the extended tongue.

28' is super long unless you are planning on 2 cars, a car and a golf cart or sleeping quarters up front. I started with a 24' for my jeep and dropped down to a 20'. I've talked to people with sleeping quarters up front and they didn't like it because if they left their vehicle in the trailer while sleeping (rest area on expressway) there were always fuel fumes.

I would plan out a floorplan. My guess is you will find you don't need that much room.

I wouldn't get a 28' bumper pull. You have to put way too much weight on the trailer axles (as opposed to a gooseneck where you can put the load over the truck). The overhang (from the rear axle to the rear of the trailer) is super long on a 28' bumper pull, which you will end up dragging in parking lots, gas stations, etc.

I paid just over $2000 for my 18' steel flatbed. A comparable aluminum was over $5500. At almost 3 times the price, I can let a trailer rust out and scrap it, buy another 6-8 years from now and still be money ahead.